1. Field of the Invention
The present invention relates to a discharge lamp starter for controlling turning on of a discharge lamp used as headlights of a vehicle and the like.
2. Description of Related Art
A conventional DC/DC converter with a flyback function is composed of a simple transformer with a flyback function as shown in FIG. 8. The DC/DC converter has the following configuration. A primary winding 101p of a transformer 101 is connected to a power supply 103 through a switching device (SW) 102, and a secondary winding 101s is connected to a capacitor 105 through a diode 104. The capacitor 105 produces an output voltage Vs across its terminals. In FIG. 8, the reference numeral 106 designates an inverter circuit, and 107 designates a high voltage generator for supplying the discharge lamp 108 with a high voltage.
FIG. 9 is a waveform chart illustrating currents and voltages of various portions to explain the operation of the conventional DC/DC converter. In response to the turning on of the switching device 102, a primary current Ip flows through the primary winding 101p, and the voltage VP across the switching device 102 is brought to zero. Thus, the core of the transformer stores magnetic energy {(½)LIp2)}.
Subsequently, in response to the turning off of the switching device 102, the magnetic energy stored in the core causes a current IS to flow through the secondary winding 101s, so that the capacitor 105 is charged through the diode 104, and generates the output voltage Vs across its terminals. The output voltage Vs of the flyback DC/DC converter varies in response to the load current, and cannot be determined by its primary voltage VP, primary current Ip, and the on-time TON of the switching device 102. In an ideal case, primary power=secondary power holds as follows.Vs·IS=VP·Ip
The DC/DC converter with the flyback function is advantageous to a wide range of the voltage and current as shown in FIG. 10: Before lighting the discharge lamp in which no current flows, a high voltage of about 400 V is applied thereto; immediately after the lighting, a low voltage of about 20 V and a large current of about 2.6 A are applied; and during normal lighting, a rated voltage of 85 V and a rated current of 0.4 A are applied. However, it has a problem of having a limit to reduce its size because it requires a rather large transformer core to store its magnetic energy via the primary winding, and to supply the energy to the secondary winding.
On the other hand, a DC/DC converter with a forward function as shown in FIG. 11 has a primary winding 111p of a transformer 111 connected to a power supply 113 through a switching device 112, a secondary winding 111s connected to a capacitor 116 via a diode 114 and a choke coil 115, and a diode 117 connected to the connecting point of the diode 114 and choke coil 115 in parallel with the secondary winding 111s, in which the output voltage Vs appears across the capacitor 116. In FIG. 11, the reference numeral 118 designates an inverter circuit, and 119 designates a high voltage generator for supplying a discharge lamp 120 with a high voltage.
FIG. 12 is a waveform chart illustrating currents and voltages of various portions to explain the operation of the DC/DC converter. The turning on of the switching device 112 causes the primary current Ip to flow through the primary winding 111p, and the secondary current Is to flow through the secondary winding 111s, with placing the voltage Vp across the switching device 112 at zero volt. On the secondary side, the capacitor 116, which is charged via the diode 114 and choke coil 115, generates the output voltage Vs across its ends.Vs=(N2/N1)·D·V1D=Ton/(Ton+Toff)                where                    N1: number of turns of the primary winding,            N2: number of turns of the secondary winding,            V1: voltage of the power supply 113,            Ton: on-duration of the switching device 112, and            Toff: off-duration of the switching device 112.                        
Since the transformer 111 of the DC/DC converter with a forward function need not store the magnetic energy in the core, the size of the core can be small. However, to output a high voltage of about 400V, it must increase the number of turns of the secondary winding 111s. For example, to boost the power supply voltage of 12 V to 400 V, the numbers of turns of the two types of the DC/DC converter transformers are as follows:
The transformer of the DC/DC converter with the flyback function: primary winding 7T; and secondary winding 42T.
The transformer of the DC/DC converter with the forward function: primary winding 7T; and secondary winding 233T.
Thus, the latter transformer becomes bulky owing to the large-size secondary winding 111s because both the transformers require the secondary winding with a large diameter to carry a large current of 2.6 A, and the secondary winding 111s is 233/42=15.6 times greater in size than the secondary winding 101s. In addition, since the latter transformer generates a voltage with a large amplitude alternating between 0 V and 400 V across the secondary winding 111s, it is necessary for the choke coil 115a to have a large capacity to smooth the voltage, making it difficult to use the transformer of the DC/DC converter with the forward function as a discharge lamp starter.
As examples of a DC/DC switching power supply that employs the two types of the transformers, there are Japanese patent application laid-open (utility model) No. 1-76185 and Japanese patent application laid-open No. 11-356046. The former is basically a forward type power supply that utilizes the flyback energy residual in the core as an auxiliary power supply. The latter is basically a flyback type constant voltage power supply trying to reduce the size of the core. Thus, they are proposed to as a power supply to recover the residual energy, or as a constant voltage power supply.